It is common knowledge that an aircraft under lifting conditions sheds vorticity rearwardly from the wing. Under most flight conditions a strong swirling vortex is shed from the vicinity of each wing tip. These two vortices rotate in opposite directions. Downstream of the aircraft the air swirls in planes transverse to the direction of flight. There is kinetic energy associated with the transverse motion of the body of air downstream of the aircraft which is imparted by the passage of the aircraft. The aircraft does work on the air so as to impart this kinetic energy. The work done by the aircraft to produce the kinetic energy imparted to the air downstream is related to the product of the velocity of the airplane and the so-called induced drag of the airplane. This induced drag, namely the drag associated with the production of kinetic energy in crossflow planes downstream of the aircraft, is a significant portion of overall airplane drag.
The induced drag is related directly to the crossflow kinetic energy downstream of the aircraft. The transverse motion of the air far downstream of the aircraft can be related directly to the spatial distribution of trailing vorticity using the well-known Boit Sarart Law. Thus, there is a direct relationship between the distribution of trailing vorticity and the induced drag that is well known. Changes to the airplane which produce changes to the trailing vorticity also change the induced drag. If the change is such that the kinetic energy in planes transverse to the direction of flight is reduced, then the induced drag of the aircraft will also be reduced.
A number of conventional apparatus and methods have been proposed to reduce the induced drag. For example, it is known that increasing the span of the wing decreases induced drag. Winglet devices which extend above or below the wing tips have also been known to reduce induced drag.
Other conventional devices for reducing wingtip vortices have attempted to change the swirl pattern of the trailing vortices. For example, rotating devices positioned aft of the wingtips have been disclosed by Patterson in U.S. Pat. No. 4,533,101, in which a rotor is positioned in the crossfield flow of the vortex and is driven mechanically in a rotational direction opposite to the vortex swirl. Furthermore, Patterson, in U.S. Pat. No. 3,984,070, discloses a non-rotating device which has blades rigidly mounted in a direction which is transverse to the airstream.
Other conventional apparatus have attempted to generate a flow of a fluidized medium which either opposes or aids the rotational flow of the vortices. For example, in U.S. Pat. No. 2,477,461, by Lee, there is disclosed a device mounted at an aircraft wingtip which generates a rotational flow of a pressurized fluid to either oppose or aid the rotation of the tip vortex. Furthermore, Haney, Jr., in U.S. Pat. No. 3,596,854, discloses a wingtip chamber which gathers high pressure air that travels spanwise around the wingtip and causes this air to swirl inside the chamber and to exit the chamber where it interacts with the wingtip vortices to reduce induced drag.
Another example of conventional apparatus for reducing wingtip vortices includes Erwin, U.S. Pat. No. 3,997,132, which discloses the mounting of a jet engine at an aircraft wingtip to reduce the vortices generated thereat.
On the other hand, Reighart, in U.S. Pat. No. 3,934,844, discloses a vortex generator which produces lift for supporting an aircraft in flight.
In U.S. Pat. No. 2,485,218, by Shaw, there is disclosed a propeller which is mounted near an aircraft wingtip and which is mechanically driven in a rotational direction opposite to the wingtip vortices in order to reduce induced drag.
U.S. Pat. No. 2,927,748 Griswold, relates to an airfoil incorporating a blowing jet to control front and rear stagnation points.
U.S. Pat. No. 1,846,562 Longo et al, discloses a small rotating wing device mounted at the rear of the main wing for rotation about a spanwise axis, the intent of this being to increase lift.
An article in Sept. 1, 1986 Aviation Week and Space Technology, entitled "Turbines Recover Power by Dissipating Induced Drag from Wingtip Vortices" shows turbine blades which recover power from the wing tip vortex.
Other patents pertaining to wingtip mounted airfoil devices include French Patents 1,456,026 and 57,646.